Trends toward shorter cooking times, consumer demand for safety, and willingness to use litigation are increasing the pressure on the food industry to reduce risks in the food chain. Meats are of particular concern because they are easily contaminated with microorganisms and are an ideal environment for growth of bacteria. Pathogens such as Salmonella, Campylobacter, Listeria, Clostridium, Escherichia coli 0157:H7, and the like can be present. Salmonella and Campylobacter jejuni are the leading causes of bacterial diarrhea. Listeria ingestion results in a high mortality rate. Escherichia coli 0157:H7 is also particularly severe and the number of incidences are increasing. The primary source of 0157:H7 infection is through beef products; however, several recent outbreaks occurred by ingestion of acidic foods, fruits and vegetables and other sources not normally associated with food poisoning, indicating that 0157:H7 is particular hardy. Today's systems to control pathogenic bacteria still result in periodic food safety problems. Only continued analysis and control (Hazard Analysis Critical Control Point HACCP!) as implemented by the Food and Drug Administration) and a multifaceted approach will allow a reasonable risk. The Food Safety and Inspection Service (FSIS)-approved antimicrobial treatments include hot water, steam and organic acids, such as lactic acid (up to 2.5%).
The use of organic acids, such as lactic acid, for decontamination of carcasses has been extensively studied because they reduce bacterial counts and are safe. The drawbacks of organic acid sprays is that high concentrations of the acids should not be used because of loss of sensory quality. Discoloration and the threshold for tasting the acid begins at about two percent.
Lactic acid has been used as an antimicrobial agent for treating animal carcasses. See U.S. Pat. No. 5,178,890, issued Jan. 12, 1993 to van den Niewelaar, et al. for "Method for Improving the Bacteriological Quality of Slaughtered Poultry"; Grau, F. H. (1986), "Microbial Ecology of Meat and Poultry," Advances in Meat Research, Vol. 2, pp 1-47; and U.S. Pat. No. 5,093,140 issued Mar. 3, 1992 to Watanabe for "Aqueous Bactericide for Animal Treatment." Application of lactic acid to meats causes a pH drop which results in death and sublethal injury to microorganisms (Anderson, M. E. and Marshall, R. T. 1989!, "Interaction of concentration and temperature of acetic acid solution on reduction of various species of microorganisms on beef surface," Journal of Food Protection 52(5):312-315). The pH remains low for a relatively short time because of the natural buffering in meat as discussed above. After spraying of hot calf carcasses with 1.25% (v/v) lactic acid, a surface pH fall of more than three units has been found. However, after 72 hours the pH had returned to its initial value. Repeating a lactic acid treatment of broiler carcasses neither decreased the surface pH further nor enhanced the bacteriostatic and bactericidal effects. See Woolthuis, C. H. J. and Smulders, F. J. M. (1985), "Microbial Decontamination of Calf Carcasses by Lactic Acid Sprays," J. Food Protection 48(10)832-837;
At high levels of initial contamination acceptable concentrations of lactic acid may not effect marked microbial lethality as theorized by Baird-Parker (Baird-Parker, A.C. 1980! "Organic Acids," in Microbial Ecology of Foods. L Factors affecting life and death of microorganisms," J. H. Silliker et al. (eds.), Academic Press, New York, pp. 126-135). When the initial contamination was low, the lethality effect of lactic acid on aerobic colony counts, though significant, did not exceed two or exceptionally three log.sub.10 cycles. (See: Snijders, J. M. A. et al. 1979!, "Dekontamination schlachtwarmer Rinderkorper mit organishen Sauren," Fleischwirtschaft 59:656-663; Smulders, F. J. M. and Woolthuis, C. H. J. 1983!, "Influence of two levels of hygiene on the microbiological condition of veal as a product of two slaughtering/processing sequences," J. of Food Protection 46:1032-1035; Smulders, F. J. M. and Woolthuis, C. H. J. 1983!, "The immediate and delayed microbiological effects of lactic acid decontamination of calf carcasses. The influence on conventionally boned versus hot boned and vacuum packaged cuts," J. of Food Protection 48:838-847; Smulders, F. J. M. and Kortenknie, F. 1985!, "Control of the bacteriological condition of calf brain. II. Effect of lactic acid decontamination and frozen storage," Int. J. Food Microbiol. 2:293-299; Van Netten, P. et al. 1984!, "A note on catalase-enhanced recovery of acid injured cells of gram negative bacteria and its consequences for the assessment of the lethality of L-lactic acid decontamination of raw meat surfaces," J. Applied Bacteriology 57:169-173; Gill, C. O. and Penney, N. 1985!, "Modification of in-pack conditions to extend the storage life of vacuum packed lamb," Meat Science 14:43-60.) However, after storage of lactic acid-treated meat surfaces a so-called delayed bacteriostatic effect was found (Smulders and Woolthuis 1985!, supra). It has also been found that lamb cuts treated with 5% lactic acid and vacuum packaged in foil laminate remained unspoiled for 12 weeks of chilled storage (Gill and Penney 1985! supra).
Lactic acid is a natural metabolite of mammalian muscle tissue and has been generally recognized as safe (GRAS) by the FDA for human consumption. Sprays at less than or equal to 2.5 percent (by weight lactic acid have been approved for antimicrobial treatment of carcasses. High molecular weight polymers of lactic acid (polylactic acid) can be formed into food packaging materials. These materials biodegrade to form lactic acid, and their use for packaging of foods has also been generally recognized as safe by the FDA. See, e.g. Conn, R. E., et al. (1995), "Safety assessment of polylactide (PLA) for use as a food-contact polymer," Food and Chemical Toxicology 33(4)273-283.
Polylactic acid has been used for a number of purposes including biodegradable implants for wound healing, timed release vehicles for drugs, fertilizers and the like, films, and grocery bags. These usages generally specify high molecular weight (e.g. above about 8,000 D) polymers.
Polylactic acid up to a molecular weight of about 15,000 D is produced by heating and condensation of lactic acid. Depending on their molecular weight, these materials can be gel-like or resinous products. Low molecular weight lactic acid materials having molecular weights up to about 15,000 daltons (D) can be prepared by condensing free (monomeric) lactic acid with or without catalysts. Classic polymerization of lactic acid to high molecular weight polymers (above about 15,000 M.W.) is a three-step process: 1) polycondensation to low molecular weight material; 2) depolymerization and cyclic dimerization to lactide; and 3) repolymerization of the lactide to high molecular weight polylactic acid (PLA) by a ring-opening reaction.
U.S. Pat. No. 5,357,034 issued Oct. 18, 1994 to Fridman et al. entitled "Lactide Polymerization" teaches making a polylactic acid composition with molecular weight between 300 and 500 by heating 85-90 weight percent lactic acid in aqueous solution to 115.degree. C. to 125.degree. C. to remove water, and then further heating to 170.degree. C. to 175.degree. C. to form the polylactic acid over a period of 5 to 8 hours. This material was then further polymerized to form high molecular weight polylactides.
U.S. Pat. No. Re. 35320 issued Aug. 27, 1996 to Kinnersley, et al. for "Method for Regulating Plant Growth" and divisional patent thereof, U.S. Pat. No. 5,238,841, discloses heating lactic acid to 100.degree. C. under vacuum for 21/2 hours to form oligomers of lactic acid having up to six molecules. Such compositions having up to six molecules were also formed by heating the dimer of L-lactic acid under reduced pressure. The dimer was obtained by hydrolysis of L-lactide. The compositions are used in aqueous solutions of 1-1000 ppm.
U.S. Pat. No. 5,274,127 issued Dec. 28, 1993 to Sinclair et al. for "Lactide Production from Dehydration of Aqueous Lactic Acid Feed" teaches removal of water from lactic acid (88%) by heating at 120.degree. C. to 185.degree. C. under nitrogen bubbling. At various (unspecified) times, aliquots were removed and analyzed, and included oligomers of up to four molecules as well as lactide. At 150.degree. C. (90 torr), the composition contained 27.8 weight percent monomers, 27.8 weight percent dimers, 20.2 weight percent trimers, 10.3 weight percent tetramers and 8.6 weight percent lactide. The object of this process was to maximize lactide production.
U.S. Pat. No. 4,801,739 issued Jan. 31, 1989 to Franz et al. for "Oligomeric Hydroxycarboxylic Acid Derivatives, Their Production and Use," discloses distilling lactic acid under argon atmosphere to 160.degree. (presumably degrees C.) for varying periods of time up to eight hours, to produce product having 12 or more lactic acid units.
None of the foregoing references teach useful heat-treated lactic acid compositions having average molecular weights less than about 700 D, and consisting of a mixture of lactic acid molecules and ester complexes of lactic acid molecules containing two to no more than about ten molecules per complex. Nor do these references teach dilute aqueous solutions having concentrations less than about 10 weight percent of the heat-treated lactic acid. Nor do any of the foregoing references teach uses of such compositions, particularly uses of such compositions as antimicrobial agents for cleaning surfaces such as meat carcasses.